docs: def diffuser_curved(Di1, Di2, l):
      r'''Returns loss coefficient for any curved wall pipe expansion
      as shown in [1]_.
  
      .. math::
          K_1 = \phi(1.43-1.3\beta^2)(1-\beta^2)^2
  
      .. math::
          \phi = 1.01 - 0.624\frac{l}{d_1} + 0.30\left(\frac{l}{d_1}\right)^2
          - 0.074\left(\frac{l}{d_1}\right)^3 + 0.0070\left(\frac{l}{d_1}\right)^4
  
      .. figure:: fittings/curved_wall_diffuser.png
         :scale: 25 %
         :alt: diffuser curved; after [1]_
  
      Parameters
      ----------
      Di1 : float
          Inside diameter of original pipe (smaller), [m]
      Di2 : float
          Inside diameter of following pipe (larger), [m]
      l : float
          Length of the curve along the pipe axis, [m]
  
      Returns
      -------
      K : float
          Loss coefficient [-]
  
      Notes
      -----
      Beta^2 should be between 0.1 and 0.9.
      A small mismatch between tabulated values of this function in table 11.3
      is observed with the equation presented.
  
      Examples
      --------
      >>> diffuser_curved(Di1=.25**0.5, Di2=1., l=2.)
      0.2299781250000002
  
      References
      ----------
      .. [1] Rennels, Donald C., and Hobart M. Hudson. Pipe Flow: A Practical
         and Comprehensive Guide. 1st edition. Hoboken, N.J: Wiley, 2012.
      '''
      beta = Di1/Di2
      phi = 1.01 - 0.624*l/Di1 + 0.30*(l/Di1)**2 - 0.074*(l/Di1)**3 + 0.0070*(l/Di1)**4
      return phi*(1.43 - 1.3*beta**2)*(1 - beta**2)**2